In testing work pieces with penetration media, such as powders capable of appearing fluorescent or also black powder or magnetic powder, i.e., conducting tests with a powder material for surface tears which thus highlighted upon examination under certain conditions and made more evident and detectable, CD cameras are used as the image recording units in automatic systems.
The CD camera, together with a downstream on-line computer, evaluates the magnetic powder data obtained from selected sample surface segments at a set algorithm.
The sample whose surface is normally evaluated for error indications for 100% with the human eye, is subdivided into segments for the evaluation of the automatic tear defect evaluation. The surface segments are selected for relevance of existing surface tears so that only the critical segments of the surface of the sample must be examined to ensure that they are free of tears. Otherwise, the examination of all of the surface of each sample would be unduly burdensome and time-consuming.
Generally, the number of the evaluated surface segments which are evaluated for tear indications determines the number of cameras.
The CD camera evaluates the tear indications with a luminosity threshold which assumes that all parameters relevant for an indication are kept as constant as possible with respect to luminosity. A parameter fluctuation resulting in luminosity changes causes a fluctuation of defect recognizability with the threshold of the camera.
Such systems have been described in publications, e.g., in German Patent Publication No. P 44 38 509.90 which is expressly referred to herein.
Conventional systems with a surface evaluation segment by segment are installed in large automobile component manufacturing plants for example. In these circumstances, a total of 18 interesting segments per part are selected from the total amount of surface segments. Each selected segment is examined with a camera for indications of tears in conjunction with the magnetic powder test.
The parts come out of production at a rate of about 18 seconds each. This means that every part must be evaluated for defects and must be conveyed into and out of the observation station within 18 seconds. This was achieved herein, as in all similar cases until now, in that 4 or 5 cameras evaluate the part in one station and in that the next 4 or 5 segments are evaluated in the next station, etc., i.e., by utilizing a plurality of examining stations. This means that the part to be examined is magnetized in an input station at a rate of one every 18 seconds and is then conveyed at the 18-second rate on a conveyor belt from the initial testing station with 4 cameras to the next, following testing station. At each testing station, 4 segments of the sample are the evaluated for indications of tears.
The cost of such a system is comparatively high, e.g., in view of the multitude of testing stations and number of cameras required in each testing station.
Furthermore, it is necessary to position the part to be examined on a pallet in such manner that all relevant surface segments can be covered by cameras. It is also necessary that the pallets be positioned in each testing station with a precision to within 1/10th of a mm so that substantially the entire potential camera surface may be utilized. With each camera, the segment is set down so that on the surface on the one hand as many surfaces as possible can be evaluated, but on the other hand so that error messages, provoked by edges on the work piece may be ignored so that false interpretations may be avoided. This means that the less exact the positioning is, the smaller is the surface which can be used for surface indications after taking into account all tolerances.
In the above example, the weight of the parts to be tested (drag bearing) is approximately 15 kg. Thus, it is a disadvantage that the known pallets are expensive in construction, while a change in the pallet directly restricts the work piece surface which can be examined.
It is another disadvantage of this system that the pallets together with the work pieces to be tested must be transported by means of relatively expensive mechanical equipment from camera station to camera station.
There furthermore exists a system for the examination by the human eye in which the part is moved. The parts are currently examined on round tables on a rotating table top in the following manner. The work piece to be tested is introduced into the magnetizing station and is set on a receiving device on the table for that purpose. Following magnetization, the part, still lying in the receiving device, is conveyed to the observation point, is checked there and the sorting into good/bad (approved/rejected) is initiated. The receiving device present is then moved in steps to the unloading/reloading station and is conveyed from there again into the magnetizing station.
The same problems as for the first example apply, e.g., expensive construction of the parts involved in the system such as the tables.